This invention relates to digital synthesizers, and more particularly to techniques for mitigating spur generation in such synthesizers for applications in radar.
Direct Digital Synthesizers (DDS) are known in the art, and are described, for example, in xe2x80x9cA Digital Frequency Synthesizer,xe2x80x9d Joseph Tierney et al, IEEE Transactions on Audio and Electroacoustics, Col. AU-19, No. 1, pages 48-57, March 1971; and xe2x80x9cA High Purity, et al, IEEE International Frequency Control Symposium, pages 207-211, 1993.
The current generation of DDSs provides the flexibility to generate both fine frequency selection and the pulse compression modulation for a radar exciter. However, the DDS can not provide the spectral purity required for high performance radars.
Prior attempts to clean up the spectrum of the DDS used for channel selection have used filters and frequency dividers. If only fine frequency is needed, a narrowband variable center frequency filter can be used to filter the signal from the DDS to produce a spectrally pure signal. Unfortunately, the narrowband filter will severely degrade the pulse compression modulation which has a bandwidth wider than the narrowband filter. Also, frequency dividers reduce the DDS signal bandwidth by a corresponding amount and thus, reduce its usefulness in a wideband application. Thus, the fidelity of the current generation of DDSs are not adequate to meet the requirements of high performance radars when used in a conventional manner in the radar exciter.
An aspect of the invention is to control a DDS so as to repeat the same sequence/waveform pulse-to-pulse, and to compensate-the resulting transmit waveform for the phase discontinuity due to the DDS repeating the sequence. In this way, the important elements of the spectrum needed for radar operation are not degraded by the less than perfect DDS.
Thus, in accordance with one aspect of the invention, a method is described for providing a pulsed radar exciter signal, in a radar exciter including a direct digital synthesizer (DDS). The method includes
resetting the DDS pulse-to-pulse to provide a pulsed DDS signal having pulse-to-pulse phase discontinuities due to the resetting of the DDS.
In accordance with a further aspect of the invention, the method can include shifting the phase of the pulsed DDS signal pulse-to-pulse by a phase shift that is held constant over a pulse to correct the pulse-to-pulse phasing of the DDS signal.
In accordance with another aspect of the invention, a pulsed radar exciter is described, which includes a direct digital synthesizer (DDS) for generating a pulsed DDS signal. A controller resets the DDS for each pulse to restart operation. A discrete phase modulator generates a phase shift signal that is held constant over a pulse to correct the pulse-to-pulse phasing of the DDS signal. A mixer combines the DDS signal and the phase shift signal to produce a composite signal which is corrected to provide a high purity signal. Some applications may not require the correction of the phase shift pulse-to-pulse and will not require the use of a discrete phase modulator.